Information recording medium with pits edges shifted in a step-wise fashion

ABSTRACT

In an information recording medium from which information recorded on each pit is reproduced by an optical detection system that obtains a reproduced signal corresponding to each pit by scanning the information recording medium along pit rows by a light beam, data of high recording density can be accurately reproduced by a simple arrangement by shifting in a step-wise fashion edge positions of an information pit from a predetermined reference position in response to digital information to be recorded within a range corresponding to a predetermined shift period smaller than a transition period of a reproduced signal determined in response to a transfer characteristic of the optical detection system. Therefore, a nonlinear intersymbol interference can be reduced without emphasizing a noise component.

This is a continuation of application Ser. No. 08/133,124 filed asPCT/JP93/00186, Feb. 15, 1993, now abandoned.

TECHNICAL FIELD

The present invention relates to information recording medium such as anoptical disk or the like and an information medium recording apparatusand a reproducing apparatus suitable for recording or reproducinginformation on or from this information recording medium.

BACKGROUND ART

In a conventional optical disk used in a CAV (constant angular velocity)mode, a servo byte interval is cyclically provided at a predeterminedposition of each track. A clock pit for generating a reference clock anda wobbled pit for effecting tracking are formed in this servo byteinterval. A reference clock (channel clock) is generated in accordancewith the clock pit and information is recorded digitally by a pit whoselength is an integral multiple of the period of this reference clock.Further, in the system such as a CD (compact disc) that is used in a CLV(constant linear velocity) mode, although there exists no clock pit, alength of the recorded pit and the pit interval are selected so as tobecome a length (9 kinds of lengths ranging from about 0.9 to 3.3 μm inthe case of CD) of an integral multiple of the reference clock (channelclock). Then, a clock is reproduced by using the clock pit and recordedinformation is sampled at the unit of bits.

In a video disc which is the same optical disk, a video signal isrecorded and reproduced by a difference of a pit considerably smallerthan that of the CD.

This fact will be described with reference to a signal recorded on aportion of a radius 55 mm in the CAV mode. In the video disk, abrightest portion in a video signal is recorded as a signal of 9.3 MHzand a darkest portion is recorded as a signal of 7.6 MHz, and thesesignals are equivalent to 1.075 μm and 1.316 μm on a disk having aradius of 55 mm, respectively. It is a well-known fact that, if the diskthus recorded is reproduced, then a very beautiful picture isreproduced. Considering that the change of brightness of 128 gradationscan be expressed in this picture, then this means that the period of thepit is recorded on the disk in the gradation of 128 or more and thenreproduced therefrom. That is, the change of fine pit length and pitinterval of

(1.316 μm-1.075 μm)÷128=0.002 μm is reflected on the video signal.

With respect to the change of the pit length, the reason that theminimum unit of the change of the pit length must be increased to be 0.3μm regardless of the fact that the very small change can be recorded ismainly assumed to be such that a recording and reproducing method is notoptimum.

The present applicant has previously proposed, as Japanese patentapplication No. 3-167585, a method in which digital information isrecorded by shifting the position of a front or rear edge of aninformation pit in a step-wise fashion from a predetermined referenceposition in response to recording information. According to thisrecording and reproducing method, since the change of the position ofthe pit length and pit edge can be detected with a very high accuracy,it becomes possible to record information by a very small change that isconsidered to be impossible. As a result, the recording with higherdensity than ever can be realized.

FIG. 33 shows a principle of recording information by shifting in astep-wise fashion the position of the edge as the present applicant haspreviously proposed. As shown in the figure, a recording signal (FIG.33B) that is PWM-modulated is generated in response to recording data.Then, a pit (FIG. 33A) corresponding to the length in zero-cross isformed. With this arrangement, the position of the edge of the pit ischanged in a step-wise fashion from the position indicated by areference clock (FIG. 33C). Data of 8 stages (3 bits) from 0 to 7 can berecorded per edge in response to the amount of such change.

FIG. 34 shows a principle of reproducing the signal thus recorded. Abinary RF signal (FIG. 34B) is obtained by considerably amplifying an RFsignal (FIG. 34A) reproduced from the information recording medium.Because the clock pits are formed on the disc in which information isrecorded, a reference clock (FIG. 34C) is generated on the basis of theclock pits, and further a sawtooth wave signal (FIG. 34D) is generatedin synchronism with this reference clock. Then, the position of the edgeof the information pit is detected by detecting a timing at which thesawtooth wave signal and the binary RF signal cross each other.

However, in the previously-proposed arrangement, there occurs anintersymbol interference between the adjacent edges. Further, if therecording density is increased, there is then the problem that theaccurate reproduction becomes impossible.

It is considered to use an equalizer in order to reduce the influence ofthe intersymbol interference. For example, the reproduced RF signal isspaced apart by a constant distance Δ by using 3-tap equalizer and thensampled three times. Then, three values are processed by a linearcalculation. An impulse response in this case is expressed as:

    h(t)=δ(t)-κ{δ(t+Δ)+κ(t-Δ)}

Therefore, its frequency response is expressed as follows:

    H(f)=1-κcos(2πΔf)

By properly selecting Δ and κ, the influence of the intersymbolinterference can be reduced while the signal component of the highfrequency region can be emphasized.

However, since there is performed the linear calculation, thisprocessing cannot be perfectly applied to a nonlinear intersymbolinterference. Also, if a value of a coupling coefficient κ is increasedin order to increase a degree of eliminating the intersymbolinterference, a noise component of the high frequency region isemphasized, which presents a contrary effect.

Further, in the previously-proposed arrangement, when the position ofthe edge is detected, the sawtooth wave is generated and the timing atwhich the edge is produced is detected from the sawtooth wave.Therefore, the arrangement for reading this timing at which the edge isproduced becomes complicated. There is then the problem that it becomesimpossible to accurately detect such timing.

Furthermore, in the previously-proposed arrangement, influences such asan amplitude fluctuation peculiar to the optical disk and a fluctuationof the bias component are not taken into consideration at all. There isthen the drawback that correct data cannot be read out due to thesefluctuations.

DISCLOSURE OF INVENTION

In view of the aforesaid situation, according to the present invention,data recorded with high density can be accurately reproduced by a simplearrangement. Also, a nonlinear intersymbol interference can be reducedwithout emphasizing a noise component.

An information recording medium according to the present invention ischaracterized in that, as shown in FIGS. 1 and 2, for example, the edgeposition of an information pit is shifted in a step-wise fashion from apredetermined reference position in response to digital information tobe recorded within a range corresponding to a predetermined shift periodsmaller than a transition period of a reproduced signal determineddepending on a transfer characteristic of an optical detection systemwhich obtains a reproduced signal corresponding to each pit by scanningthe medium along pit rows by a light beam.

Further, there is added an education pit, inserted into pit rows, whoseedge position is shifted in a step-wise fashion from a predeterminedreference position in response to education data and which is set inadvance in response to the insertion position.

Further, there is added a reference pit which is inserted into the pitrows of information pits and the shift amount of the edge position ofwhich is set to be a minimum value.

Furthermore, there is added a reference pit which is inserted into pitrows of information pits and the shift amount of the edge position ofwhich is set to be a maximum value.

An information recording apparatus according to the present inventionincludes, as shown in FIG. 32, for example, recording means forrecording digital information on an information recording medium fromwhich recorded information is reproduced by an optical detection systemthat scans the medium along the pit rows by a light beam by shifting ina step-wise fashion the edge position of information pit from apredetermined reference position within a range corresponding to apredetermined shift period smaller than a transition period of areproduced signal determined depending on a transfer characteristic ofthe above-mentioned optical detection system.

According to the present invention, there is provided an informationreproducing apparatus in which, as shown in FIG. 5, for example,recorded information is reproduced from an information recording mediumin which digital information is recorded by shifting in a step-wisefashion the edge position of the information pit from a predeterminedreference position within a range corresponding to a predetermined shiftperiod smaller than a transition period of a reproduced signaldetermined depending on a transfer characteristic of an opticaldetection system that scans the medium by a light beam along pit rows.This information reproducing apparatus includes clock generating meansfor generating a clock that is synchronized to the reference position inphase on the basis of the reproduced signal from the optical detectionsystem, a level detecting means for detecting the reproducing level inthe transition period of the reproduced signal, and a judging means forjudging recorded information corresponding to the shift amount of theedge position of the information pit on the basis of the reproducinglevel.

Further, the clock generating means generates a clock at a timingcorresponding to a center of the shift period.

Further, the clock generating means generates a clock synchronized tothe reference position in phase on the basis of the reproduced signalobtained through the optical detection system from the reference pitthat indicates a predetermined reference position recorded in a servoarea of the information recording medium.

Further, the level detecting means is formed of an A/D converter circuitthat detects the reproducing level by analog-to-digital converting thereproducing signal at a sampling timing defined by the above-mentionedclock.

Further, the judging means judges recorded information corresponding torespective shift amounts of the adjacent edge positions of the pit rowdirection of the information pit on the basis of reference pointsdefined by the reproducing levels sequentially detected by the leveldetecting means from education pits in which education datapre-determined in response to the insertion position in the pit row ofthe information pit are set as respective shift amounts of the edgepositions adjacent in the pit row direction and information pointsdefined by the reproducing levels sequentially detected from theinformation pit.

Further, the judging means judges a pair of recorded informationscorresponding to respective shift amounts of a pair of edge positionsadjacent in the pit row direction of the information pit on the basis ofreference points defined by a pair of reproducing levels detected by thelevel detecting means from education pits in which a pair of educationdata pre-determined in response to the insertion position are set aarespective shift amounts of the pair of edge positions and informationpoints defined by the pair of reproducing levels detected from theinformation pit.

Further, of a pair of reproducing levels detected from the education pitin which a pair of education data are set, an address defined when onereproducing level is set to a high-order address and the otherreproducing level is set to a low-order address is used as a referencepoint. There is provided memory means in which a pair of education dataare stored in this reference point as decoded data. The above judgingmeans uses an address which is defined when one reproducing level of apair of reproducing levels detected from the information pit is set to ahigh-order address and the other reproducing level is set to a low-orderaddress as an information point and judges a pair of decoded data storedin the memory at its address corresponding to this information point asrecording information.

Furthermore, the judging means judges, of decoded data stored in therespective reference points of the memory means, decoded data stored inthe reference point nearest to the information point as the recordinginformation.

Furthermore, there is provided mapping means using as a reference pointan address of the memory means defined by the reproducing level detectedfrom the education pit in which education data is set and which effectsa mapping processing for storing the education data in this referencepoint as decoded data.

Furthermore, the mapping means stores decoded data stored in thereference point nearest to each storage point in respective storagepoints other than reference point of respective storage points of thememory means.

Further, there is provided a bias eliminating means which subtracts thereproducing level detected from the reference pit in which the shiftamount of the edge position is set to a minimum value from thereproducing level detected by the level detecting means.

Further, the bias eliminating means includes a defect eliminatingfunction to subtract from the reproducing level an average value ofrespective values except a maximum value and a minimum value of aplurality of reproducing levels detected from a plurality of referencepits in which the shift amount of the edge position is set to a minimumvalue.

Further, there is provided a gain adjusting means for adjusting a gainof a reproducing level detected by the level detecting means such thatthe reproducing level detected from the reference pit in which the shiftamount of the edge position is set to a maximum value becomes apredetermined target value.

Further, the gain adjusting means includes a defect eliminating functionto adjust the gain of the reproducing level such that an average valueof respective values except a maximum value and a minimum value of aplurality of reproducing levels detected from a plurality of referencepits in which the shift amount of the edge position is set to a maximumvalue becomes a predetermined target value.

In the information recording medium according to the present invention,the edge position of the information pit is shifted in a step-wisefashion from the predetermined reference position in response to digitalinformation to be recorded within a range corresponding to apredetermined shift period smaller than a transition period of thereproducing signal determined depending on the transfer characteristicof the optical detection system.

Therefore, according to the information reproducing apparatus of thepresent invention, the recorded information corresponding to the shiftamount of the edge position of the information pit can be judgedreliably by detecting the reproducing level at one sampling timing inthe transition period of the reproduced signal. Thus, it becomespossible to accurately reproduce information of high recording densityby a simple arrangement.

Furthermore, by judging the recorded information corresponding to therespective shift amounts of the edge positions adjacent in the pit rowdirection of the information pit on the basis of reference pointsdefined by the reproducing levels sequentially detected from theeducation pit in which education data pre-determined in response to theinsertion position relative to the pit row of the information pit areset as respective shift amount of the edge positions adjacent in the pitrow direction and information points defined by the reproducing levelssequentially detected from the information pit, it becomes possible toreduce a nonlinear intersymbol interference to thereby effect theaccurate decoding without emphasizing a noise component.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram used to explain arrangements of a data area and aservo area of an information recording medium of the present invention,

FIG. 2 is a diagram used to explain an example of an arrangement of aninformation pit in an information recording medium of the presentinvention,

FIG. 3 is a diagram used to explain an example of an arrangement of aninformation pit in an information recording medium of the presentinvention,

FIG. 4 is a diagram used to explain phases between tracks of theinformation recording medium of the present invention,

FIG. 5 is a block diagram showing an arrangement of an embodiment of anoptical disk reproducing apparatus to which an information reproducingapparatus of the present invention is applied,

FIG. 6 is a block diagram showing an example of an arrangement of a PLLcircuit 7 in the embodiment of FIG. 5,

FIG. 7 is a block diagram showing an example of an arrangement of atwo-dimension decoder in the embodiment of FIG. 5,

FIG. 8 is a block diagram showing an example of arrangements of a biaseliminating circuit 10 and the two-dimension decoder 11 in theembodiment of FIG. 5,

FIG. 9 is a diagram used to explain an intersymbol interference betweenadjacent edges,

FIG. 10 is a diagram used to explain a principle of a mapping ofreference points on a RAM 23 of FIG. 8,

FIGS. 11A and 11B are diagrams used to explain functions indicative ofthe influences of the adjacent edges,

FIGS. 12A and 12C are diagrams used to explain a relationship betweenthe influence of the adjacent edges and the linear density,

FIGS. 13A-13I are timing charts used to explain operation of a servoarea in the embodiment of FIG. 8,

FIG. 14 is a diagram used to explain the mapping of reference points inthe RAM 23 of FIG. 8,

FIG. 15 is a diagram used to explain the mapping of reference pointsrelative to other storage points in the RAM 23 of FIG. 8,

FIG. 16 is a diagram used to explain the mapping state of referencepoints relative to all storage points in the RAM 23 of FIG. 8,

FIG. 17 is a flowchart to which references will be made in explainingthe procedure of the mapping processing done by a controller 15 of FIG.8,

FIGS. 18A-18G are timing charts used to explain operation in a data areain the embodiment of FIG. 8,

FIG. 19 is a diagram used to explain an error rate realized by theembodiment of FIG. 5,

FIG. 20 is a diagram used to explain an example of other arrangement ofthe information recording pit,

FIG. 21 is a diagram showing an example of an arrangement of a servoarea provided when the information recording pit is constructed as shownin FIG. 20,

FIG. 22 is a block diagram showing an arrangement of other embodiment ofthe optical disk reproducing apparatus to which the informationreproducing apparatus of the present invention is applied,

FIGS. 23A-23I are timing charts used to explain operation of theembodiment of FIG. 22,

FIGS. 24A and 24B are diagrams used to explain the change of level ofthe outputs of gain variable amplifiers 63, 66 in FIG. 22,

FIG. 25 is a diagram used to explain an error produced situationrealized by the embodiment of FIG. 22,

FIG. 26 is a block diagram showing an arrangement of a furtherembodiment of the optical disk reproducing apparatus to which theinformation reproducing apparatus of the present invention is applied,

FIG. 27 is a diagram used to explain operation of a defect eliminatingcircuit 73 of FIG. 26,

FIG. 28 is a block diagram showing an example of an arrangement of thedefect eliminating circuit 73 of FIG. 26,

FIG. 29 is a diagram of a table used to explain a calculation in acontroller 104 of FIG. 26,

FIG. 30 is a diagram used to explain the influence exerted by the defectof the outputs of the gain variable amplifiers 63, 66 of FIG. 26,

FIG. 31 is a block diagram showing an example of other arrangement forcalculating a minimum distance between education data and reproduceddata,

FIG. 32 is a block diagram showing an arrangement of an embodiment of anoptical disc manufacturing apparatus to which an information recordingapparatus of the present invention is applied,

FIGS. 33A-33C are diagrams used to explain a data recording method in aconventional optical disk, and

FIGS. 34A-34D are diagrams used to explain a data reproducing method inthe conventional optical disk.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will hereinafter be described withreference to the drawings.

FIG. 1 shows an example of a fundamental format of an optical disk towhich the information recording medium of the present invention isapplied.

In this embodiment, on a reflection type (pits are formed on areflection surface of a light beam in the form of physical concaveportions or convex portions) optical disk 1 having a diameter of 120 mm,there are recorded pit rows at the track pitch of 1.6 μm in the CLVmode. All informations are recorded as shift amounts of 8 stages of edgepositions of the front (leading edge) and a rear (trailing edge) of thepit disposed at every constant period of 1.67 μm. A unit shift amount Δthat is one unit of this shift amount is set to 0.05 μm.

Because each 3-bit information can be recorded by the shift amount ofthe 8 stages of the edge positions of each pit arranged as describedabove, the linear recording information density in the pit row directionis 0.28 μm/bit, which is more than twice that of the present CD system.

Incidentally, in the CD system, even when the linear velocity is set toan upper limit of 1.2 m/s, 8-bit data to be recorded is converted into17-bit channel bit of 14-bit information bit and 3-bit margin bit by anEFM (Eight to Fourteen Modulation) and then recorded. Taking this EFMinto consideration, the linear recording information density is about0.6 μm/bit. That is to say, since the shortest pit of about 0.9 μmcorresponds to the channel bit of 3 bits, we have:

    (0.9÷3)×(17÷8)=about 0.6 μm/bit

As shown in FIG. 2, although the edge position of the pit recorded onthe optical disk 1 is shifted in a stepwise fashion from the referenceposition of the center of the pit in response to digital information tobe recorded, a shift period Ts (=Δ×7) thereof is set in a rangecorresponding to a period smaller than the leading edge period tr ortrailing edge period tf which is a transition period (period other thanthe stationary state in which data becomes 0 level or saturated level)of an RF signal (reproduced signal) determined depending on the transfercharacteristic of the optical detection system.

The above-mentioned RF signal is output from a pickup 3 of a reproducingapparatus which will be described later on, and the transition period isdetermined by the transfer characteristic of the pickup 3. It iscustomary that a transfer characteristic of the optical system isdefined by an MTF (Modulation Transfer Function) which is an absolutevalue of its transfer function (OTF: Optical Transfer Function). ThisMTF is determined depending on a numerical aperture NA of a lens and awavelength λ of a laser.

If the unit shift amount Δ is shifted at the unit amount smaller than0.05 μm within the above-mentioned shift period Ts, then a recordingdensity can be increased more.

The RF signal is A/D-converted by a sampling clock SP synchronized inphase with the reference position of the center of the pit thusrecorded, whereby reproducing levels L0 to L7 corresponding to the shiftamounts 0 to 7 of the edge position of the pit can be obtained. Thecondition that the reproducing levels L0 to L7 can be detected at onesampling timing in the transition period of the RF signal as describedabove is p1 shift period Ts ≦ transition period (leading period tr ortrailing period tf)

As a sampling timing done by the sample clock SP, it is desired to use atiming corresponding to the center of the shift period Ts. With thistiming, it becomes possible to detect the reproducing level over thewhole range of the transition period of the RF signal. Further, whilethe so-called reflection type optical disk 1 in which the pits areformed on the reflection surface of the light beam as the physicalconcave or convex portions will be described in this embodiment, thepresent invention is not limited thereto and can be applied to aso-called MO (Magneto Optical) disk in which pits are formed by thepartial inversion of a magnetic polarity of a magneto-optical film.

Information recorded on the optical disk 1 is sampled at the unit of 3bits and then recorded on n'th bit as recording data an and bn. FIG. 3shows that state in which the front end edge of the pit is set to anyone of 8 shift positions from 0 to 7 in response to recording data an.Similarly, the position of the rear end edge also is set to any one of 8shift positions from 0 to 7 in response to recording data bn. The pitchA of each shift position is 0.05 μm as described before. As a result,when the recording data an and bn are formed at the shift position 0,each pit has a shortest length LP=0.5 μm.

Referring back to FIG. 1, a servo area formed of 5 pits is inserted intodata area formed of 44 data pits in which data is recorded. Of 5 pitsfor servo, two pits are used as education pits P1, P2 and the remainingthree pits are used as reference pits P3 to P5. The position of thefront end edge of the education pit P2 in the left-hand side of thefigure is set to any position M of the shift positions of 8 stages from0 to 7. Further, the position of the rear end edge in the right-handside of the figure is set to any position N of the shift positions of 8stages from 0 to 7.

The position M of the front end edge of the education pit P2 and theposition N of the rear end edge thereof are set in the servo area by aregular combination so as to provide different combinations. Morespecifically, in the first servo area, M and N are set to (0, 0) and(0, 1) in the next servo area. Similarly, there are set regularcombinations of (0, 2), (0, 3), . . . , (7, 6), (7, 7). Therefore, in 64(=8×8) servo areas, there are prepared all possible combinations ofpositions of the front end edge and the rear end edge of the educationpit P2.

Incidentally, the education pit P1 becomes a dummy pit in this case.More specifically, the education data is not formed at the respectiveends of the pit P2 and can be formed at the respective ends of the pitP1 from a theory standpoint. However, with such arrangement, since thepit adjacent to the left-hand side of the pit P1 is the data pit of thedata area so that the edge position is changed in response to the data.As a result, a degree in which the education pit P1 interferes with theedge of the data area side is changed depending on the value of theeducation data. Therefore, it becomes impossible to constantly formeducation data under the constant state as will be described later on.Therefore, according to this embodiment, it is preferable to formeducation data on the edges of the respective ends of the education pitP2, not on the respective ends of the pit P1. With the above-mentionedarrangement, the edges of the reference pit P4 adjacent to the rear endedge of the education pit P2 and education pit P1 adjacent to the frontend edge thereof both remain constant (not changed) as (0, 0) so that,when education data of the education pit P2 is read out, such educationdata is constantly affected by a constant intersymbol interference.Therefore, it is possible to obtain a constant pattern.

The reference pits P3 to P5 are pits that are used to obtain data ofreference position of (0, 0) and (7, 7). From a theory standpoint, thisreference data can be formed at edges of the respective ends of, forexample, pit P5. However, with the above-mentioned arrangement, thedegree of interference from adjacent data region is changed inaccordance with recording data similarly to the case that has beendescribed so far with respect to the education pit. Therefore, it ispreferable that the reference position data is not formed at the rearend edge of the reference pit P5 in the right-hand side of the figuresimilarly to the embodiment.

FIG. 4 is a diagram used to explain a planar structure of the opticaldisk 1 in brief. Since the signal recorded at the track pitch of 1.6 μmis recorded in the CLV mode, the phases of the pit positions are notmatched between the adjacent tracks, and this signal is recorded withdifferent phases as shown in the figure.

FIG. 5 is a block diagram showing an arrangement of an embodiment of anoptical disk reproducing apparatus to which the information reproducingapparatus of the present invention is applied. The optical disk 1 isrotated by a spindle motor 2. On this optical disk 1, there is recordedinformation on the basis of the principle shown in FIGS. 1 and 2. Morespecifically, digital information is recorded by shifting in a step-wisefashion at least one position of the front end edge and rear end edge ofthe information pit from a predetermined reference position. Then, servoareas are formed on this optical disk 1 at a constant period and alsothe education pits P1, P2 and the reference pits P3 to P5 are formedthereon. It is needless to say that a data pit is formed on the dataarea.

The pickup 3 irradiates the optical disk 1 with a laser beam toreproduce a signal recorded on the optical disk 1 from a reflected-backlight thereof. The RF signal output from the optical pickup 3 isamplified by a head amplifier 4 and then supplied to a focus trackingservo circuit 5, an APC circuit 6 and a PLL circuit 7. The focustracking servo circuit 5 effects the focus error signal and trackingcontrol on the input signal. Also, the APC circuit 6 effects the servocontrol such that intensity of laser light radiated on the optical disk1 becomes constant.

The PLL circuit 7 is adapted to extract a clock component from the inputsignal. While the PLL circuit that is generally utilized in the CDsystem reproduces a clock by using all RF signals, in the case of thisembodiment, a clock is reproduced by using only the RF signal at theportion of the servo area. That is to say, since the portion of theservo area is not modulated by the recorded data, the clock can bereproduced stably without being affected by the influence of therecorded data at all.

FIG. 6 is a block diagram showing an arrangement of the PLL circuit 7that can realize such function. As shown in the figure, when a servoarea pattern judging circuit 171 detects a pattern, which is consideredto be a servo area, from the RF signal, there is generated a servo areadetection pulse. There is the possibility that the same pattern as theservo area will appear in the data area. It is to be understood thatthis pulse is not always correct. However, assuming that this pulse iscorrect, then a lock detection circuit 172 supplies a reset pulse to acounter 173 to reset the counter 173.

If this pattern is a correct servo area, then the detection pulse willbe output at the same timing. The lock detection circuit 172 detectsthis fact to determine whether or not the PLL circuit 7 is in the lockedstate.

If it is determined that the pulse is not the correct detection pulse,then the lock detection output is not produced after a constant time waspassed. Therefore, the above-mentioned operation is repeated until thelocked state is presented.

After the servo area is detected correctly, the counter 173 is reset atthe correct timing so that a timing at which the next servo area appearscan be coarsely predicted by decoding a count value of the counter 173.

Using this principle, a timing at which a particular pit existing withinthe servo area appears is generated from the count value of the counter173 and is then supplied to an AND gate 176 as a gate signal.

In order to eliminate the influence from the data recorded on both sidesof the servo area, the timing of the gate signal is adjusted such that apit at the central portion of the servo area is selected as much aspossible.

Of RF signals differentiated by the differentiating circuit 174 andwhose zero-cross points are detected by a zero-cross detection circuit,a signal that is passed through an AND gate 176 become a phase comparingpulse and is then supplied to a sample and hold circuit 177.

The sample and hold circuit 177 detects a time difference (phase error)of a clock that counts up the counter 173 and a particular pit existingin the servo area on the optical disk 1 by momentarily sampling andholding a sawtooth wave generated by a sawtooth wave generating circuit178 on the basis of the counter 173 at a timing of a particular pit.

This phase error signal that had been passed through a filter 179 is fedback to a VCO (voltage controlled oscillator) as a drive voltage,thereby constructing a PLL loop that can constantly hold a particularpit existing in the servo area on the optical disk 1 and the clock in acorrect phase relationship.

A sampling clock SP, a clock A, a clock B, a clock RA, a clock RB, aclock TA and a clock TB that are held at predetermined phaserelationships shown in FIG. 13 are generated by decoding the output ofthe counter 173 by a decoder 181. These clocks are supplied to an A/Dconverting circuit 9, a bias eliminating circuit 10 and a two-dimensiondecoder 11 as shown in FIG. 5.

A spindle servo circuit 8 in FIG. 5 controls the spindle motor 2 suchthat the drive voltage of the VCO in FIG. 6 becomes a constant valueconstantly. Thus, the optical disk 1 is rotated at a constant angularvelocity.

On the other hand, the RF signal output from the head amplifier 4 isinput to the A/D converting circuit 9, in which it is A/D-converted intodigital data (reproducing level) indicative of the levels of 256 stagesof 8 bits. The 8-bit data is supplied to a bias eliminating circuit 10,in which it is eliminated in bias component and then supplied to atwo-dimension decoder 11 and a controller 15. The controller 15 iscomprised of a CPU that effects a variety of calculations, a program ROMin which there are stored programs that are executed by this CPU, etc.,to thereby execute a mapping processing or the like which will bedescribed later on.

The two-dimension decoder 11 decodes a signal supplied thereto from thebias eliminating circuit 10 and supplies its output to a 6-8 bitconversion circuit 12. The 6-8 bit conversion circuit 12 accumulates 4sets of input 6-bit data and converts and outputs 3 sets of 8-bit datato an error correction circuit 13. The error correction circuit 13corrects error of input data and outputs corrected data to a D/Aconverting circuit 14. The D/A converting circuit 14 converts the inputdata into an analog signal and outputs the same into an analog audioamplifier.

The two-dimension decoder 11 is constructed as, for example, shown inFIG. 7. More specifically, the 8-bit reproduced level data supplied fromthe bias eliminating circuit 10 is sequentially delayed by delaycircuits 21 and 22. Then, data output from the delay circuit 21 of thefirst stage and the data delayed by the delay circuit 22 of thesucceeding stage are output to a RAM 23 as address data. The RAM 23reads out 6-bit decoded data written in addresses corresponding toaddress data supplied thereto from the delay circuits 21 and 22 andoutput the same to the 6-8 bit conversion circuit 12.

FIG. 8 shows more fully arrangements of the bias eliminating circuit 10and the two-dimension decoder 11. That is to say, in this embodiment,the 8-bit data output from the A/D converter circuit 9 is supplied tolatch circuits 31 and 32 and also supplied to latch circuits 41 and 43that construct the bias eliminating circuit 10 together with subtractingcircuits 42, 44. The subtracting circuit 42 subtracts data latched inthe latch circuit 41 from the data latched in the latch circuit 31, andthe subtracting circuit 44 subtracts the data latched in the latchcircuit 43 from the data latched in the latch circuit 32.

Outputs of the subtracting circuits 42, 44 are supplied to the RAM 23 as8-bit high-order address and 8-bit low-order address. Further, the latchcircuits 33, 34 latch 8-bit education data supplied thereto from thesubtracting circuits 42, 44 at a predetermined timing and output thelatched education data to the controller 15. The controller 15 isprogrammed in advance so that education data are formed as pattern dataso as to be mapped on the RAM 23.

Operation of the above-mentioned embodiment will be described and priorto such description, a principle for reading out the shift position ofthe information pit in this embodiment will be described.

Assuming that pits are spaced apart with a sufficient distance and thatan intersymbol interference from adjacent pits is ignored, then outputdata (reproducing levels) of the A/D converting circuit 9 in the shiftperiods of the front end edge and the rear end edge of the n'th pit areVa(n) and Vb(n), as shown in FIG. 9. Va(n) and Vb(n) indicate the levelsof the RF signal and can be expressed by the following equations:

    Va(n)=Δrf×an+ga(bn)

    Vb(n)=Δrf×bn+gb(an)

Here, Δrf represents values which result from multiplying the unit shiftamount Δ with a constant k, and ga(bn) and gb(an) represent nonlinearfunctions expressing the intersymbol interference between the two edges.These values are increased as the recording density is increased (i.e.,as the two edges become close). The decoding of data is such that thesesimultaneous equations are solved and the recorded signals an, bn arecalculated from the observed Va(n) and Vb(n).

Incidentally, the above-mentioned nonlinear function is coarselyobtained on the basis of a line spread function (Line spread function) fshown in FIG. 9. This line spread function f is determined by adistribution of intensity of a reflected light from the reflectionsurface of the optical disk 1. The line spread function is describedmore in detail in "APPLIED OPTICS/Vol.26, No.18/15 September 1987 P.3961to P.3973".

These signals an and bn can be grasped as a pattern recognition problemon a two-dimensional space. More specifically, for all combinations of(an, bn), there are carried out the calculation of the above-mentionedequations. Then, when the resultant Va(n) is plotted as a value of the Xaxis and Vb(n) is plotted as a value of the Y axis on the two-dimensionspace, then values that Va(n) and Vb(n) can take are expressed asinformation points shown in FIG. 10. On this two-dimension plane, thefunctions ga(bn) and gb(an) expressing the influence of the intersymbolinterference are expressed as position distortions of the informationpoints. That is to say, when the functions ga(bn) and gb(an) are 0 (whenthere occurs no intersymbol interference), information points arelocated at the positions (lattice points) (reference points) at whichlines shown by broken lines in FIG. 10 cross each other. However, inactual practice, there occur intersymbol interference functions gb(an),ga(bn) which become monotone increase functions as, for example, shownin FIG. 11. As a result, as shown in FIG. 10, information points shownby solid circles in the figure are displaced from the lattice points(reference points).

Because this displacement occurs due to the intersymbol interference,this displacement becomes large as the intersymbol interference becomeslarge. More specifically, FIG. 12A shows a displacement (distortion)obtained when the linear recording density is 0.32 μm/bit and FIGS. 12Band 12C show distortions obtained when the wiring recording density is0.36 μm/bit or 0.46 μm/bit. Therefore, it is to be understood that thedistortion becomes large as the linear recording density becomes small(as the density is increased).

More specifically, education data recorded on the above-mentionededucation pit P2 is reproduced and the information point defined by thereproducing level is mapped on the RAM 23 as a reference point as shownby the solid circles in FIG. 10. Then, information point that isobtained when data is read out from the data pit is plotted on the RAM23 and it is determined that the nearest reference point is a referencepoint corresponding to the information point. Then, the edge position(an, bn) indicated by the reference point is output as the edge positionof the read-out information pint.

Mapping operation of reference points on the RAM 23 will be describedinitially with reference to a timing chart of FIG. 13.

The pickup 3 reproduces a signal recorded on the optical disk 1. Thisreproduced RF signal is supplied through the head amplifier 4 to the A/Dconverter circuit 9, in which it is A/D-converted at a timing of aleading edge of a sampling clock SP (FIG. 13C). Digital data output fromthe A/D converter circuit 9 is supplied to the latch circuits 31, 32,41, 43. These latch circuits are respectively supplied with a clock A(FIG. 13D), a clock B (FIG. 13E), a clock RA (FIG. 13H) and a clock RB(FIG. 13I).

The clock A, the clock B, the clock RA, the clock RB, a clock TA (FIG.13F) and a clock TB (FIG. 13G) are those generated by the PLL circuit 7.As is clear from FIG. 13, the clock A and the clock B are generatedimmediately after the timings at which the front end edge and the rearend edge of each pit are sampled. Also, the clocks RA and RB aregenerated at the timing in which the reference position data (0, 0) ofthe reference pits P3 of respective servo areas are generated.

Accordingly, in the latch circuits 41 and 43, there are latched thereference position data (0, 0) of the front edge and the rear edge ofthe reference pit P3 in the previous servo area. When the education dataof the front end edge and the rear end edge of the education pit P2 arelatched by the latch circuits 31 and 32, the subtracter 42 subtracts thelatched data of the latch circuit 41 from the latched data of the latchcircuit 31. Similarly, the latched data in the latch circuit 43 issubtracted from the data latched by the latch circuit 32.

More specifically, the subtracting circuits 42 and 44 output a leveldifference between the position M (M is any one of from 0 to 7) of theeducation pit P2 and the position 0. Further, the subtracting circuit 44outputs a level difference between the position N (N is any one ofvalues from 0 to 7) and the position 0. In this way, a DC component(bias component) of the reproduced signal is eliminated by subtractingthe levels at the position 0. Then, the data whose DC component iseliminated is supplied to the latch circuits 33 and 34, respectively.The latch circuits 33 and 34 latch this data at the timing at which theclocks TA and TB are input. In other words, the latch circuits 33 and 34output the education data whose DC component is eliminated to thecontroller 15.

It is needless to say that absolute level of data at each shift positioncan be latched without subtracting the values of the reference positiondata (0, 0). However, if so, then the absolute level of each shiftposition is changed due to the fluctuation of the disk 1 and the opticalsystem or the like. Therefore, it becomes impossible to judge each shiftposition. For this reason, it is preferable to reduce the influence ofthe fluctuations of the disk 1 and the optical system by subtracting thevalues of the reference position data (0, 0) as described above.

The controller 15 effects the mapping operation on the RAM 23 on thebasis of the information points defined by the two coordinates asreference points as education data input thereto from the latch circuit33, i.e., a low-order address in FIG. 8 and as education data inputthereto from the latch circuit 34, i.e., a high-order address in FIG. 8.

When the mapping operation is carried out in accordance with educationdata from 64 servo areas, as shown in FIG. 14, 64 reference points aremapped on the RAM 23 at its predetermined storage points.

Then, the controller 15 calculates the distances of the respectivestorage points on the RAM 23 relative to the storage points in which the64 storage points are stored. That is, as shown in FIG. 15, for example,distances of storage points m1 to m17 relative to a storage point mi inwhich a reference point (0, 7) is stored are calculated. Similarly,distances of the storage points m1 to m17 relative to a storage point mjin which a reference point (1, 7) is stored are also calculated. Then,in the respective storage points, there are stored the same data as thereference point stored in the nearest storage point of the storagepoints at which the reference points are stored.

Since the quantization bit number per sample in the A/D convertercircuit 9 is 8 bits, the output thereof has the levels of 256 stages.Accordingly, the RAM 23 has 256 addresses as the abscissa and ordinateaddresses. In other words, the RAM 23 is comprised of 256×256 storagepoints. Of these storage points, the reference points are stored inpredetermined reference points as shown in FIG. 14.

With respect to other storage points in which the reference point is notstored, distance between them and the storage points at which thereference points are already stored are calculated and the same data asthe reference point stored in the nearest storage point is stored ineach of the storage points. In the embodiment shown in FIG. 15, of thestorage points m1 to m17, the storage points m1 to m9 are nearest to thestorage point mi (reference point (0, 7)) and the storage points m10 tom17 are nearest to the storage point mj (storage point (1, 7)).Accordingly, the data of the reference point (0, 7) is written in thestorage points m1 to m9. That is to say, these storage points are usedas an area A (0, 7) corresponding to the reference point (0, 7). On theother hand, data at the reference point (1, 7) are written in thestorage points m10 to m17. That is to say, these storage points are setto the area A (1, 7) corresponding to the reference point (1, 7).

As described above, data at the reference points are written in the256×256 storage points so that areas on the RAM 23 corresponding to therespective reference points are presented as shown in FIG. 16. In thestorage points included in respective areas A (i, j) are stored data atthe reference points (i, j).

A flowchart of the mapping processing done by the above-mentionedcontroller 15 is illustrated in FIG. 17.

As described before, the position M of the front end edge and theposition N of the rear end edge of the education pit P2 of each servoarea of the optical disk 1 are set to different combinations. Theseeducation data (M, N) are regularly set in response to the insertionposition relative to the data area in a predetermined order. Morespecifically, the education data are set to (0, 0) in the first servoarea, and (0, 1) in the next servo area. In a like manner, the educationdata are set to (0, 2), (0, 3), . . . , (7, 6), (7, 7).

Accordingly, in step SP shown in FIG. 17, education data (0, 0)corresponding to the reference position data (0, 0) are sequentiallyfrom the incoming servo areas. Then, when the first servo area isdetected, the processing proceeds to step SP2, whereat M and N are set.Then, the processing proceeds to step SP3. In step SP3, decoded data (0,0) is stored by using the address (storage point) designated by theeducation data (0, 0) on the RAM 23 as the reference point (0, 0).Thereafter, steps SP3 to SP8 are repeated similarly, thereby storingdecoded data (0, 1), . . . , (7, 7) in addresses (storage points)designated by the education data (0, 1), . . . , (7, 7) on the RAM 23.

Thereafter, the processing proceeds to step SP9, whereat decoded data(i, j) are stored in storage points other than the reference points (i,j) by the above-mentioned interpolation calculation processing.

The above-mentioned mapping processing is executed by the controller 15as the initialization operation each time different optical disks 1 areloaded onto the reproducing apparatus.

Operation in the data area will be described with reference to a timingchart of FIG. 18. In response to pit rows shown in FIG. 18A, an RFsignal shown in FIG. 18B is input to the A/D converter circuit 9. Then,the levels of the front end and rear end edges of each pit are sampledin synchronism with the leading edge of a sampling clock SP (FIG. 18C).As shown in FIGS. 18A and 18B, the phase of the RF signal is changed inresponse to the edge position of the pit.

Then, this sampling clock SP is generated during this edge shift periodso that the edge shift position can be detected as the change of thelevel of the RF signal.

Data of the front end edge of the data pit latched by the latch circuit31 is calculated in level difference relative to data of the position 0latched in the latch circuit 41 is calculated and then supplied to theRAM 23 as a low-order address (as address of the abscissa in FIG. 10).Similarly, data of the rear end edge of the data pit latched in thelatch circuit 32 is subtracted in level of the data of the position 0latched in the latch circuit 43 and eliminated in DC component,whereafter it is supplied to the RAM 23 as a high-order address (addressof the ordinate in FIG. 10). The RAM 23 reads out and outputs decodeddata stored in the addresses defined by the abscissa and the ordinate.As the decoded data, there are written reference points of educationdata by the above-mentioned mapping process. Accordingly, there areselectively output data (an, bn) of the reference points that are closeto the information points.

The 6-bit decoded data (an, bn) output from the two-dimension decoder 11shown in FIG. 5 is supplied to the 6-8 bit conversion circuit 12, inwhich it is converted into 8-bit data. More specifically, when an audiosignal, for example, is recorded on the optical disk 1, the audio signalis corrected in error at the unit of 8 bits. However, as describedabove, according to this embodiment, totally 6 bits of 3 bits (shiftpositions of 8 stages) of the front end edge and 3 bits (shift positionsof 8 stages) of the rear end edge are taken as a fundamental unit torecord data. More specifically, upon recording, data segmented at theunit of 8 bits is converted into data segmented at the unit of 6 bits inaccordance with a predetermined system and data is recorded on theoptical disk 1. Therefore, data of the 6-bit unit is converted intooriginal data of the 8-bit unit by the 6-8 bit conversion circuit 12.

This bit conversion circuit is operated such that, after 4 sets of dataof the 6-bit unit were decoded, these data are collected and 3 sets ofdata of the 8-bit data are output.

The 8-bit unit data inversely converted by the 6-8 bit conversioncircuit 12 is supplied to the error correction circuit 13,error-corrected by the error correction circuit 13 and then supplied tothe D/A converter circuit 14, in which it is D/A-converted. Then, thisanalog signal is amplified by an analog audio amplifier, not shown, andemanated from a speaker or the like as a reproduced sound.

FIG. 19 shows an error rate of data obtained by the decoding under thecondition that the bias eliminating circuit 10 is dis-energized in theabove-mentioned embodiment. As shown in the figure, it is to beunderstood that, as compared with the conventional decoding method(method described in Japanese patent application NO. 3-167585 previouslyproposed by the present applicant) using the sawtooth wave, the errorrate can be reduced when the two-dimension decoding is carried out byusing the reference points mapped on the above-mentioned RAM 23. Also,it is to be appreciated that a large effect can be achieved as thelinear recording density is increased.

While data an and bn are recorded on a pair of front edge and rear endedge of one pit as shown in FIG. 1 as described above, data an and bncan be recorded on opposing edges of the adjacent pits as, for example,shown in FIG. 20. In this case, with respect to the education data andthe position reference data in the servo area, the education data andthe position reference data are recorded on opposing edges of the twopits as shown in FIG. 21. In this embodiment, education data (M, N) isrecorded on opposing edges of the two pits P1 and P2, and the positionreference data (0, 0) is recorded on opposing edges of the referencepits P3, P4. The position reference data (7, 7) is recorded onrespective opposing edges of the reference pits P4 and P5.

In this case, the clock A and the clock B are respectively generated atthe rear edge and the front edge of the pit as shown in FIGS. 18F and18G.

A distance between the information point obtained from the reproduceddata and the reference point obtained from the education data is notstored in the RAM 23 in advance but can be calculated at every time.However, if so, then the rapid judgement becomes impossible. Therefore,it is preferable to write such distance data in the RAM 23 in advancelike the embodiment.

While the bias eliminating circuit 10 is located between the A/Dconverter circuit 9 and the two-dimension decoder 11 in the embodimentshown in FIG. 5, a gain adjusting circuit 5 can be inserted thereintoexcept the bias eliminating circuit 10. FIG. 22 shows an embodiment ofthis case. More specifically, the output of the latch circuit 42 issupplied to a gain variable amplifier 63 and also to a latch circuit 61,in which it is latched by a clock KA. An output of the latch circuit 61is supplied to a subtracting circuit 62, in which a difference betweenit and a predetermined target amplitude is calculated. Then, an outputof the subtracting circuit 62 is supplied to a gain variable amplifier63.

Similarly, an output of the subtracting circuit 44 is supplied to a gainvariable amplifier 66 and also to a latch circuit 64. Data latched bythe latch circuit 64 at the timing of the clock KB is supplied to asubtracting circuit 65, in which a target amplitude value supplied froma circuit, not shown, is subtracted from it and then supplied to a gainvariable amplifier 66.

The gain variable amplifiers 63, 66 can be formed of ROMs. Outputs ofthe subtracting circuits 42 and 62 (44 and 65) are input to the ROMs asaddresses and data corresponding to those addresses are read outtherefrom.

The output of the gain variable amplifier 63 is supplied to the RAM 23and the latch circuit 33 and the output of the gain variable amplifier66 is supplied to the RAM 23 and the latch circuit 34. That is to say, again adjusting circuit 60 is connected to the rear stage of the biaseliminating circuit 10. A rest of the arrangements is similar to thoseof FIGS. 5 and 8.

The embodiment of FIG. 22 will be described next with reference to atiming chart of FIG. 23. In the embodiment of FIG. 2, education data isdisposed on respective opposing edges of the education pits P1, P2unlike the embodiments shown in FIGS. 1 and 13. The reference positiondata (0, 0) is recorded on opposing edges of the reference pits P3, P4and the reference position data (7, 7) is recorded on the opposing edgesof the reference pits P4 and P5 (FIG. 23A).

An RF signal shown in FIG. 23B is obtained in response to the educationpits P1, P2 and the reference pits P3 to P5. This signal isA/D-converted by the A/D converter circuit 9 at a timing of a samplingclock SP shown in FIG. 23C. This data is latched in the latch circuit 31at a timing of a clock A (FIG. 23D) and then latched by the latchcircuit 32 at a timing of a clock B (FIG. 23E). Further, the data islatched by the latch circuit 41 at a timing of clock RA (FIG. 23F) andthen latched by the latch circuit 43 at a timing of a clock RB (FIG.23G).

Then, the subtracting circuit 42 subtracts the output of the latchcircuit 41 from the output of the latch circuit 31,and the subtractingcircuit 44 subtracts the output of the latch circuit 43 from the outputof the latch circuit 32. In this way, data that can be prevented frombeing affected by the DC component can be obtained (the reference pointcan be disposed at the position of the lattice point at which thestraight lines shown by the broken lines cross each other in FIG. 14)similarly as described above.

In this embodiment, the latch circuit 61 latches the output of thesubtracting circuit 42 at a timing of a clock KA (FIG. 23H). That is,reference data 7 that is recorded at the rear end edge of the referencepit P4 is latched in the latch circuit 61. A pre-determined targetamplitude is subtracted from the output of the latch circuit 61 by thesubtracting circuit 62. A difference therebetween is supplied to thegain variable amplifier 63. The gain variable amplifier 63 adjusts thegain of the signal supplied thereto from the subtracting circuit 42 inresponse to the signal supplied thereto from the subtracting circuit 62.That is to say, therefore, the signal output from the gain variableamplifier 63 is set such that its position of the abscissa directionindicated by the reference point (7, 7) becomes a target amplitude inFIG. 10.

Similarly, a latch circuit 64 latches the output of the subtractingcircuit 44 at a timing of a clock KB (FIG. 23I). More specifically, theposition reference data 7 that is recorded at the front end edge of thereference pit P5 is latched in the latch circuit 64. Data latched by thelatch circuit 64 is subtracted in target amplitude by a subtractingcircuit 65 and then supplied to a gain variable amplifier 66. The gainvariable amplifier 66 adjusts the gain of the signal supplied theretofrom the subtracting circuit 44 in response to the signal supplied fromthe subtracting circuit 65. Thus, the signal output from the gainvariable amplifier 66 is adjusted so that the position thereof in theordinate direction shown by the reference point (7, 7) becomes a targetamplitude position that is pre-determined in FIG. 10.

As described above, by adjusting the gain by the gain adjusting circuit60, the reference position (7, 7) shown in FIG. 14 can constantly belocated at a predetermined position. Therefore, even when the opticaldisk 1 has a local fluctuation of characteristic, data can be read outaccurately.

FIG. 24 shows the output of the gain variable amplifier 63 (or 66). FIG.24A shows such output obtained when the output of the subtractingcircuit 62 is not supplied to the gain variable amplifier 63, and FIG.24B shows such output obtained when that output is supplied thereto. Itis to be understood that the level fluctuation is suppressed if the gainis adjusted by the output from the subtracting circuit 62.

FIG. 25 shows measured results of the number of errors of C1 when theerror correction method used in the CD system is applied in theembodiment of FIG. 22. In the figure, solid circles represent measuredresults obtained when the outputs of the latch circuits 41, 43 aresupplied to the subtracting circuits 42, 44 and the outputs of thesubtracting circuits 62, 66 are not supplied to the gain variableamplifier 63. In the figure, solid circles show measured resultsobtained when the outputs of the latch circuits 41, 43 are supplied tothe subtracting circuits 42, 44 and the outputs of the subtractingcircuits 62, 65 are supplied to the gain variable amplifiers 63, 66. Itis to be understood that the latter can reduce the number of resultanterrors. Also, it is to be understood that the number of errors cansatisfy the CD standard regardless of the fact that the linear recordingdensity is twice that of the CD.

As described above, the reproduced data is processed on the basis of thedata recorded on the education pits P1, P2 or the reference pits P3through P5 so that, if a dropout or the like occurs in these referencedata, data cannot be read out accurately. In order to prevent thisdefect, a circuit is constructed as, for example, shown in FIG. 26. Morespecifically, in this embodiment, the latch circuits 31, 32 are replacedwith FIFOs 71, 72, and the latch circuits 41, 43 are replaced withdefect eliminating circuits 73, 74. Further, the latch circuits 61 and64 are replaced with defect eliminating circuits 82, 84, and FIFOs 81,83 are inserted into the front stages of the gain variable amplifiers63, 66. A rest of arrangements is similar to that of the case of FIG.22.

More specifically, the defect eliminating circuit 73 can store therein 4blocks of, for example, data input from the A/D converter circuit 9 asshown in FIG. 27. Then, position reference data (0, 0) in respectiveblocks are compared and average value of two data except the maximum andminimum values is calculated, which then is used as data of the positionreference data (0, 0). Therefore, even if the value of positionreference data (0, 0) becomes an abnormal value due to the dropout orthe like, such abnormal value can be prevented from being used as thereference data.

This is also true in other defect eliminating circuits 74, 82, 84.

Incidentally, because the defect eliminating circuits 73, 74, 82, 84must store data of 4 blocks, the FIFIOs 71, 72, 81, 83 must are used todelay data by the delay amounts based on such data and supply the sameto the subtracting circuits 42, 44 or the gain variable amplifiers 63,66.

FIG. 28a shows an example of an arrangement of the defect eliminatingcircuit 73 (84, 82, 84). In this embodiment, data input from the A/Dconverter circuit 9 is sequentially latched in the latch circuits 91through 94 in synchronism with the clock RA. Then, the data latched inthe latch circuits 91 to 94 are read out to the data bus when gates 95to 98 are turned on. Data on the data bus are latched in latch circuits99 through 102 at a predetermined timing in synchronism with a clockoutput from a controller 104. This controller 104 is comprised of a CPUfor processing a variety of calculations, a ROM in which a program usedby this CPU is stored in advance, etc.

Data latched in the latch circuits 99 and 100 are supplied to acomparing circuit 103, in which they are compared in level. Then, asignal SAB corresponding to the compared result is supplied to thecontroller 104. The controller 104 supplies gate control signals EA, EB,EC, ED to the gates 85 to 98 from which predetermined data are output tothe data bus, and also generates clocks output to the latch circuits 99to 102. Also, the controller judges a maximum value and a minimum valueof data latched in the latch circuits 91 to 94 from the signal suppliedfrom the comparing circuit 103 on the basis of a table shown in FIG. 29.

More specifically, the controller 104 outputs a predetermined one of thegate control signals EA through ED at a predetermined timing and allowsthe latch circuits 99 and 100 to latch therein predetermined two data ofthe data latched in the latch circuits 91 to 94. The magnitude of thethus latched data is judged by the comparing circuit 103. By repeatingthis processing several times, the maximum value and the minimum valueof the data latched in the latch circuits 91 to 94 are calculated.

In FIG. 29, of latch data R_(n-1) to R_(n+2), when the left-hand sidedata on the uppermost row is larger than the right-hand side data, suchdata is expressed by a logic 1. When it is smaller, such data isexpressed by a logic 0. When it is not definite, such data is expressedby X. For example. when data R_(n-1) latched in the latch circuit 94 islarger than data R_(n) latched in the latch circuit 93, when dataR_(n-1) latched in the latch circuit 94 is larger than data R_(n+1)latched in the latch circuit 92 and when data R_(n-1) latched in thelatch circuit 94 is larger than data R_(n+2) latched in the latchcircuit 91, then the data R_(n-1) latched in the latch circuit 94becomes a maximum value.

Further, when data R_(n-1) is larger than data R_(n), when data R_(n) islarger than data R_(n+1) and when data R_(n) is larger than dataR_(n+2), data R_(n) becomes a maximum value.

Similarly, the maximum values and the minimum values will be obtainedfrom FIG. 27 hereinafter.

On the basis of the table shown in FIG. 29, the controller 104 reads outother data than the maximum and minimum values to the data bus from thedata R_(n-1), to R_(n+2) stored in the latch circuits 91 to 94 when themaximum and minimum values are detected and latch the same in the latchcircuits 101 and 102. The data latched in the latch circuits 101 and 102are added by an adding circuit 105, multiplied with a coefficient 1/2 bya multiplying circuit 106 and then supplied to a latch circuit 107, inwhich it is latched. More specifically, average value of two data exceptthe maximum value and the minimum value of the data R_(n-1) to R_(n+2)latched in the latched circuits 91 to 94 is lathed in the latch circuit107. This data is supplied to the subtracting circuit 42.

FIG. 30 shows the change of level obtained when the defect eliminatingcircuits 73, 74, 82, 84 are used (shown by A in the figure) in FIG. 26and when they are not used (shown by B in the figure). It is to beappreciated that, when the defect eliminating circuits are not used, thelevel is fluctuated in response to the defect caused by the dropout orthe like. On the other hand, it is to be understood that when the defecteliminating circuits are used, the defect is eliminated so that thefluctuation of the level is suppressed. That is to say, data can bejudged more accurately.

Furthermore, the minimum distance between the information point obtainedfrom the reproduced data and the reference point set by the educationdata can be also detected by a circuit arrangement shown, for example,in FIG. 31.

In this embodiment, the reproduced RF signal is A/D-converted by an A/Dconverter circuit 50 and then latched by latch circuits 51, 52. Thelatch circuit 51 latches data corresponding to the front edge of, forexample, the above-mentioned pit and the latch circuit 52 latches datacorresponding to the rear edge thereof. Data latched by the latchcircuits 51 and 52 are supplied to 64 correlators 53-1 through 53-64. 64education data are supplied to the correlators 53-1 to 53-64,respectively. Each of the correlators 53-1 to 53-64 calculates acorrelation between data supplied from the latch circuits 51, 52 andeducation data and outputs a calculated result to a maximum valuedetector 54. The maximum value detector 54 is formed of, for example, aWinner take All circuit and detects and outputs maximum data from 64data supplied thereto from the correlators 53-1 to 53-64.

While the CLV mode is described above, the present invention is notlimited thereto and the CAV mode also may be applied to the presentinvention. In this case, if pits are disposed so as to have a phasedisplacement of 90 degrees between adjacent tracks, then an influence ofa crosstalk between the adjacent tracks can be reduced and the highdensity on the track direction can be realized.

While all reference points corresponding to the education data aremapped onto the RAM 23 as described above, the present invention is notlimited thereto and such a variant is also possible that only referencepoints of a part thereof (e.g., 16 portions) may be mapped by educationdata and other references points may be interpolated by the calculationfrom the reference points mapped by the education data.

Finally, an embodiment of a recording apparatus for the above highrecording density optical disk 1 will be described.

In FIG. 32, an information source 201 outputs an audio signal in theform of a digital signal as a signal to be recorded. A ECC circuit 202adds an error correction code to digital audio data supplied from theinformation source 201 and outputs the same to a conversion circuit 203.The conversion circuit 203 converts input data into data of 3-bit unit.More specifically, in this embodiment, edge position of each pit is setto any one positions of from 0 to 7. The 3-bit data is required in orderto specify each edge position. The conversion circuit 203 generates this3-bit data.

A clock information generating circuit 205 generates data necessary forgenerating a clock that is required to read out data recorded in theoptical disk 1. A bias gain information generating circuit 206 generatesdata indicative of a bias point (data indicative of the reference point(0, 0) and data indicative of the fact that positions of the front endedge and the rear end edge are both 0) and data for setting a gain (dataindicative of the reference point (7, 7) and data indicative of thepositions of the front end edge and the rear end edge are both 7).

A PLL pull-in signal generating circuit 207 generates a signal that isused to pull-in the PLL. An education data generating circuit 208generates data whose edge position (an, bn) corresponds to edgepositions of (0, 0) to (7, 7). Data output from the clock informationgenerating circuit 205, the bias gain information generating circuit206, the PLL pull-in signal generating circuit 207 and the educationdata generating circuit 208 are all supplied to an adder 204, in whichthey are added with data supplied from the conversion circuit 203 (in atime division multiplexed).

An output of the adder 204 is supplied to a recording edge positioncalculating circuit 209, and an output of this recording edge positioncalculating circuit 209 is output to an edge modulating circuit 210. Anoutput of the edge modulating apparatus 210 is supplied to a masteringcircuit 211 and the optical disk 1 is formed through the processes suchas cutting, development, plating process, transfer process, aluminumevaporation, protecting film coating process or the like.

In the above-mentioned arrangement, the edge modulating circuit 210generates a timing signal of a timing corresponding to data output fromthe recording position calculating circuit 209 and outputs the same tothe mastering circuit 211.

The edge position modulating circuit 210 is arranged to generate timingsignals of timings such that the front end and rear end edge positionsof each pit are shifted from the reference position of the center ofthese pits in 8 stages in response to digital information to berecorded. In this case, the shift period Ts of the edge position of eachpit is set so as to fall within a range corresponding to a periodsmaller than a transition period (leading period tr or trailing periodtf) of the RF signal determined in response to a transfer characteristicof the optical detection system (pickup 3) on the reproducing apparatusside.

The mastering apparatus 211 cuts the photosensitive film coated on arecording master disk in response to a timing signal supplied theretofrom the edge modulating circuit 210 by a laser beam. The master diskthus cut is developed, and plated to thereby form stampers. Then, pitsformed on the stampers are transferred onto a replica. This replica istreated by an aluminum evaporation process and is coated with aprotecting film, thereby the optical disk 1 being manufactured.

As described above, according to the information recording medium of thepresent invention, since the edge position of information pit is shiftedin a step-wise fashion from the predetermined reference position inresponse to the digital information to be recorded within a rangecorresponding to the predetermined shift period smaller than thetransition period of the reproducing signal determined in response tothe transfer characteristic of the optical detection system, a recordingdensity of twice or more can be obtained as compared with theconventional CD system.

According to the information reproducing apparatus for reproducing theinformation recording medium of the present invention, since thereproducing level can be detected at one sampling timing in thetransition period of a reproducing signal, recording informationcorresponding to the shift amount of the edge position of informationpit can be judged reliably so that information of high recording densitycan be reproduced accurately by a simplified arrangement. Therefore, thearrangement thereof can be simplified and the apparatus can be madeinexpensive.

Further, since recording information corresponding to each of the shiftamounts of the edge positions adjacent in the pit rows of informationpits is judged on the basis of the reference points defined by thereproducing level sequentially detected from the education pit in whicheducation data pre-determined in response to the insertion positionsrelative to the pit rows of the information pit are set as respectiveshift amounts of the edge positions adjacent in the pit row directionand the information points defined by the reproducing level sequentiallydetected from the information pit, a non-molding intersymbolinterference can be reduced without emphasizing the noise component.Furthermore, the recording density can be increased more and the moreaccurate decoding becomes possible.

Further, since the reference points are mapped in a storage meansfashion, the edge position can be judged easily and rapidly.

Further, since the reference points are mapped from the education pitsrecorded on the information recording medium, data can be accuratelyread out without being affected by the fluctuation of the informationrecording medium.

Further, since a part of the reference points are obtained from thereferences points defined from the education pit by the calculation, thenumber of education pits recorded on the information recording mediumcan be reduced and the capacity of the information recording medium canbe utilized more effectively.

Further, since the reference point is stored in the storage pointdefined by the address corresponding to the reproducing level of theeducation pit, the reference point corresponding to the informationpoint can be judged with ease.

Further, since the reference point stored in the nearest storage pointsof the reference points defined by the address corresponding to thereproducing level of the education pit is stored in the storage pointother than storage points defined by the address corresponding to thereproducing level of the education pit, the corresponding referencepoint can be judged rapidly and the accurate decoding becomes possibleeven when the reproducing level is fluctuated somewhat by the noise.

Further, since the signal corresponding to the reference pit located atthe shift position in which the edge shift amount is smallest issubtracted from the signal reproduced from the information recordingmedium, data can be read out accurately without being affected by theinfluence of the local characteristic fluctuation of the informationrecording medium itself.

Further, since the signals correspond to the reference pits located atthe shift position at which the edge shift amount is smallest and at theshift position at which the edge shift amount is largest are subtractedfrom the signal reproduced from the information recording medium, datacan be read out accurately regardless of the respective fluctuations ofthe information recording medium and the local characteristics of theinside thereof.

Furthermore, since the education pits are formed, the recorded data canbe reproduced accurately without being affected by the fluctuations ofthe information recording medium and the characteristics of thereproducing apparatus.

Furthermore, since the education data is formed apart from the data pit,it is possible to suppress education data from being affected by therecorded data.

Furthermore, since the reference pit having the edge of the shiftposition at which the shift amount is smallest is recorded at thepredetermined position other than the data pit, it is possible torealize the information recording medium which can read out dataaccurately even when there occur fluctuations.

Furthermore, since the reference pit having the edge position of theshift position at which the shift amount is largest is recorded at thepredetermined position other than the data pit, it is possible torealize the information recording medium from which data can read outaccurately even when there occurs local characteristic fluctuation.

Furthermore, since the reference pits having the edge of the shiftposition at which the shift amount is smallest and the edge of the shiftposition at which the shift amount is largest are recorded at thepredetermined positions other than the data pit, it is possible torealize the information recording medium from which data can be read outaccurately regardless of the respective fluctuations and the localfluctuations.

In addition, according to the information recording apparatus of thepresent invention, the dynamic range of the length of the pit to berecorded is low so that the influence of the heat accumulation effectupon recording can be ignored substantially. Therefore, a satisfactorysignal characteristic can be obtained when data is recorded with thehigh recording density.

We claim:
 1. An information recording medium of the type formed by theprocess of recording digital information as pits on the informationrecording medium which can be reproduced by an optical detection systemwhich obtains a reproduced signal corresponding to each pit by scanningsaid information recording medium along pit rows, wherein theimprovement comprises forming said information recording medium by theprocess of shifting an edge position of each information pit in astep-wise fashion from a predetermined reference position in response todigital information to be recorded, wherein said digital information isrecorded as shift amounts of multiple stages of said edge positions. 2.The information recording medium according to claim 1, wherein the frontend and rear end edges of said information pit are shifted in astep-wise fashion from a predetermined reference position in response todigital information to be recorded.
 3. The information recording mediumaccording to claim 1, wherein said information pit is formed on areflection surface of an optical beam in the form of a physical concaveor convex portion.
 4. The information recording medium according toclaim 1, wherein said information pit is formed by a partial inversionof the magnetic polarity of a magneto-optical film.
 5. The informationrecording medium according to claim 1, wherein said information pit isarranged at a constant distance interval when said information recordingmedium is moved at a constant linear velocity.
 6. The informationrecording medium according to claim 1, wherein said information pit isarranged at a constant angular interval relative to a central angle whensaid information recording medium is moved at a constant angularvelocity about a predetermined rotational center.
 7. The informationrecording medium according to claim 1, wherein said predeterminedreference position is set in response to a central position of saidinformation pit.
 8. The information recording medium according to claim1, further comprising an education pit, inserted into a pit row, whoseedge position is shifted in a step-wise fashion from said predeterminedreference position in response to education data and which ispre-determined in response to an insertion position.
 9. The informationrecording medium according to claim 8, wherein a pair of edge positionsadjacent to a pit row direction of said education pit are shifted in astep-wise fashion from said predetermined reference position in responseto a pair of education data pre-determined in response to said insertionposition.
 10. The information recording medium according to claim 9,wherein all possible combinations are pre-determined in response to saidinsertion position as said pair of education data and education pitscorresponding to education data of said all combinations aresequentially inserted into and disposed at the insertion position of thepit row of said information pit.
 11. The information recording mediumaccording to claim 8, 9 or 10, wherein said education pit is located ata position at which at least one pit from said information pit isinterposed.
 12. The information recording medium according to claim 1,8, 9 or 10, further comprising a reference pit inserted into a pit rowand whose edge position shift amount is set to a minimum value.
 13. Theinformation recording medium according to claim 12, wherein shiftamounts of a pair of edge positions adjacent to the pit row direction ofsaid reference pit are both set to minimum values.
 14. The informationrecording medium according to claim 1, 8, 9 or 10, further comprising areference pit inserted into the pit row and whose edge position shiftamount is set to a maximum value.
 15. The information recording mediumaccording to claim 14, wherein shift amounts of a pair of edge positionsadjacent to the pit row direction of said reference pit are both set tomaximum values.
 16. An information recording medium comprising:arecording surface; pit rows on said recording surface; a plurality ofinformation pits disposed along each of said pit rows, each of saidinformation pits having a leading edge and a trailing edge that are eachshifted from a center reference position of said information pit by oneof a plurality of shift amounts, said shift amounts constituting digitalinformation recorded on said recording medium.
 17. The informationrecording medium of claim 16, wherein said center reference positions ofsaid information pits are disposed at constant distance intervals alongsaid pit rows.
 18. The information recording medium of claim 16, whereinsaid center reference positions of said information pits are disposed atconstant angular intervals along said pit rows.
 19. The informationrecording medium of claim 16, wherein said plurality of shift amountscomprises eight stages of edge positions shifted in a step-wise fashionthat are equally separated from each other by a predetermined unit shiftamount.
 20. The information recording medium of claim 19, wherein saiddigital information is user data.
 21. The information recording mediumof claim 20, wherein said recording surface is a reflective surface, andsaid information pits are formed as physical concave or convex portions.22. The information recording medium of claim 20, wherein said pits areformed by a partial inversion of the magnetic polarity of amagneto-optical film formed on said recording surface.
 23. Theinformation recording medium of claim 19, further comprising:aneducation pit, inserted into one of said pit rows, having a leading edgeand a trailing edge that are each shifted from a center referenceposition of said education pit by one of a plurality of second shiftamounts, said second shift amounts constituting education data.
 24. Theinformation recording medium of claim 23, wherein said second shiftamounts of said education pit are pre-determined in response to aninsertion position.
 25. The information recording medium of claim 24,wherein a pair of said information pits are disposed along the one ofsaid pit rows with said education pit disposed therebetween, the edgepositions of said pair of information pits adjacent to said educationpit in a pit row direction are shifted in a step-wise fashion from saidcenter reference position of said education pit in response to a pair ofeducation data pre-determined in response to said insertion position.26. The information recording medium of claim 25, wherein all possiblecombinations of shift amounts are pre-determined in response to saidinsertion position as said pair of education data, and education pitscorresponding to education data of said all combinations aresequentially inserted into and disposed at the insertion position of thepit row of said information pits.
 27. The information recording mediumof claim 16, further comprising:a reference pit inserted into one ofsaid pit rows and having a leading edge and a trailing edge that areeach shifted from a center reference position of said reference pit byone of a plurality of third shift amounts, said third shift amountsconstituting reference information recorded on said recording medium.28. The information recording medium of claim 27, wherein a pair of saidinformation pits are disposed along the one of said pit rows with saidreference pit disposed therebetween, the shift amounts of edge positionsof said pair of information pits adjacent to said reference pit in a pitrow direction are both set to maximum values.